(a) Field of the Invention
The present invention relates to an active matrix LCD panel and a method for fabricating the active matrix LCD panel. More particularly, the present invention relates to a channel protection type active matrix LCD panel in which a channel protection film is formed on a surface of an amorphous silicon (a-Si) active layer, and a method for fabricating the channel protection type active matrix LCD panel.
(b) Description of the Related Art
An active matrix mode liquid crystal display (LCD) device including an active element such as a thin-film transistor (TFT) has advantages of small thickness and light weight, and has been widely used as a high quality flat panel display unit. In general, the LCD device is either of a longitudinal electric field (twisted nematic: TN) mode wherein liquid crystal layer interposed between an active matrix LCD panel and a counter panel respectively mounting thereon transparent electrodes is driven by a voltage applied between the transparent electrodes, or of an in-plane switching mode where a liquid crystal is driven by comb-shaped electrodes formed on the active matrix LCD panel for generating transverse electric field which is parallel to the surface of both the panels. In either mode LCD device, attempts to simplify the fabrication process of the active matrix LCD panel have been made in order to realize a lower price.
Referring to FIG. 1 showing a typical TN mode active matrix LCD panel generally includes: gate lines 12 and drain lines 14 respectively extending in directions perpendicular to each other; pixel electrodes 10 each formed in a pixel area surrounded by these lines; and thin-film transistors (TFTs) 17 each formed in the vicinity of an intersection between the gate line 12 and the drain line 14. The source of each TFT is coupled to a gate line of next row via a gate storage capacitor having a gate storage electrode 21 and connected in parallel with the pixel capacitor formed by a liquid crystal layer. Each drain line 14 as well as each gate line 12 is protected by a protective device 18 including first and second TFTs 41 and 42 disposed in the vicinity of the electrode pad, i.e., gate terminal 15 or drain terminal 16. A channel protection film is formed on the surface of the thin-film transistor 17 in order to secure the performance thereof. An orientation film (not shown) for aligning the liquid crystal layer in a predetermined direction is formed on each of the thin-film transistor 17 and the pixel electrode 10 of the active matrix LCD panel. The liquid crystal lays is sandwiched between the active matrix LCD panel and the counter panel, on which a color filter, a common electrode, an orientation film, and the like are formed, to implement an active matrix LCD device.
A general fabrication method for such an active matrix LCD panel will be described below. First, an ITO (Indium-Tin-Oxide) film is formed by deposition onto a transparent insulating substrate, and a resist pattern is formed on the ITO film by using a first mask pattern (or a first photolithographic step using a mask pattern). The ITO film is selectively etched by using the resist pattern to form pixel electrodes. Thereafter, a metallic film such as made of Cr, Mo, or Al to be formed as gate electrodes is formed by deposition onto the transparent insulating substrate. Another resist pattern is then formed on the metallic film by using a second photolithographic technique, followed by selective etching of the metallic film to form gate electrodes.
Subsequently, a gate insulating film such as made of SiNx is formed by deposition to cover the gate electrodes, followed by selecting etching thereof to form openings therein by using a third photolithographic step. An a-Si layer is then formed thereon by deposition. Next, the a-Si layer is selectively etched by using a fourth photolithographic step to form a plurality of island a-Si layers, and a channel protection film such as made of SiNx is formed on the island a-Si layers by deposition. Using a fifth photographic process, the channel protection film is selectively etched to leave portions of the channel protection film on the island a-Si layers. Next, in order to obtain ohmic contact with the a-Si layer, an impurity-doped n+type a-Si layer is deposited thereon, and a metal such as Cr, Mo, or Al is subsequently deposited thereon. Then, a sixth photolithographic step is conducted to form source/drain electrodes of the TFTs.
In the process for fabrication of the active matrix LCD panel, six photolithographic steps using six mask patterns are conducted in total for completion of the active matrix LCD panel. In view of simplification of the fabrication process for the active matrix LCD panel, it has been proposed to reduce the large number of the photolithographic steps in the fabrication process.
Japanese Patent Laid-Open Publication No. Sho 63-218925, for example, describes a fabrication process for reducing the number of the photolithographic steps, which is described below with reference to FIGS. 2A to 2D schematically illustrating steps of the method for fabricating the TN mode active matrix LCD panel in sections. The left side of each of the figures shows the peripheral area of the panel where gate terminals are disposed, and the centers thereof show a single pixel disposed in the pixel area.
In the active matrix LCD panel described in the above-mentioned publication, first, an ITO film and a metallic film such as made of Cr, Mo, or Al are consecutively deposited onto a transparent insulating substrate 31, as illustrated in FIG. 2A, by a sputtering technique, and a resist pattern is formed thereon using a first photolithographic step. The ITO film and the metallic film are selectively etched by using the resist pattern to simultaneously form gate electrodes 32 and pixel electrodes 10.
Next, as illustrated in FIG. 2B, after a gate insulating film 34 such as made of SiNx, an intrinsic or undoped a-Si layer 35, and a channel protection film 25 such as made of SiNx are consecutively deposited, followed by selective etching of the channel protection film 25 to leave portions of the channel protection film 25 on the channel region of the undoped a-Si layer 35.
Next, as illustrated in FIG. 2C, an ohmic contact layer 36 made of impurity-doped n+-type a-Si is deposited thereon, and the ohmic contact layer 36, the undoped a-Si layer 35, the gate insulating film 34, and the upper layer gate metallic film 32b are selectively etched all together using a third photolithographic step to expose pixel electrodes 10 and electrode pads for the gate electrode 32.
Thereafter, as illustrated in FIG. 2D, a source/drain metallic film such as made of Al is formed thereon by deposition. Subsequently, the source/drain metallic film and the ohmic contact layer 36 above the channel region are selectively etched using a fourth photolithographic step, and the source/drain metallic film is configured to a specific shape, thereby completing the fabrication of the active matrix LCD panel.
In the method of the above publication, it is possible to fabricate the active matrix LCD panel in which the channel protection film 25 is formed in the channel region above the undoped a-Si layer 35 by using the four photolithographic steps. However, since the ohmic contact layer 36, the undoped a-Si layer 35, the gate insulating film 34, and the upper layer gate metallic film 32b are selectively etched all together in the single step as illustrated in FIG. 2C after the formation of the channel protection film 25, the side surfaces of the undoped a-Si layer 35 are not covered and thus not protected by the channel protection film 25.
The side walls of the undoped a-Si layer 35 not protected by the channel protection film 25 made of a fine material, such as SiNx, is subjected to ingress of impurities from the liquid crystal layer through a coarse film such as a polyimide orientation film alone by diffusion or by an electric field. This significantly deteriorates the characteristics of the TFT having source/drain regions formed in the undoped a-Si active layer 35. In order to avoid this problem, in the current active matrix LCD panel, a passivation film is formed so as to cover the side surfaces of the undoped a-Si layer 35 after the step illustrated in FIG. 2D. In such a case, another (fifth) photolithographic step should be conducted to the passivation film for exposing therethrough the gate terminals, drain terminals, and pixel electrodes. Thus, the number of photolithographic steps is five, and reduction of the number of photolithographic steps is limited.
A primary object of the present invention is to solve the above problem and provide a channel protection type active matrix LCD panel capable of covering the entire surface of the a-Si layer by the passivation film by using less number of photolithographic steps, and also to provide a method for fabricating the active matrix LCD panel.
The present invention provides an active matrix LCD panel for use in an active matrix LCD device including a transparent insulating substrate, a plurality of pixels arranged on the transparent insulating substrate and each including a pixel electrode and an associated TFT, and a passivation layer covering the plurality of pixels, the TFT having a gate electrode including a transparent conductive film and a metallic film consecutively formed on the transparent insulating substrate, a gate insulating film covering the gate electrode, an island amorphous silicon (a-Si) layer formed on the gate insulating film, and source and drain electrodes, the passivation layer covering side surface and top surface of the a-Si layer and having openings therein, the source and drain electrodes being in contact with the a-Si layer through the respective openings of the passivation layer, the pixel electrode including a transparent conductive film formed in a common layer with the transparent conductive film of the gate electrode, the source electrode being in contact with the transparent conductive film of the pixel electrode through the opening of the passivation layer.
In accordance with the active matrix LCD panel of the present invention, since the passivation layer covering the side surface and the top surface of the a-Si layer functions as a channel protection layer, the number of photolithographic steps in the fabrication process for the channel protection type LCD panel can be reduced down to four, which simplifies the fabrication process for the active matrix LCD panel.
The present invention also provides a method for manufacturing an active matrix LCD panel including the consecutive steps of:
depositing consecutively a transparent conductive film and a metallic film thereon, and selectively etching the transparent conductive film and the metallic film by a first photolithographic step using a first mask pattern to from gate electrodes and pixel electrodes;
depositing consecutively a gate insulating film and an a-Si layer thereon, and selectively etching the gate insulating film and the a-Si layer by a second photolithographic step using a second mask pattern to form island a-Si layers and gate insulating films;
depositing a passivation film thereon to cover top and side surfaces of the island a-Si layers, and selectively etching the passivation film by a third photolithographic step using a third mask pattern to form openings therein for exposing portions of the island a-Si layers and the pixel electrodes; and
depositing an electrode film thereon, and selectively etching the electrode film by a fourth photolithographic step using a fourth mask pattern to form electrodes in contact with the portions of the island a-Si layers and the pixel electrodes.
In accordance with the method of the present invention, the number of photolithographic steps can be reduced to four, which simplifies the fabrication process for the active matrix LCD panel.